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watchdog: imgpdc: Add some documentation about the timeout

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This watchdog hardware can be configured in terms of power-of-two
clock cycles. Therefore, the watchdog timeout configured by the user
will be rounded-up to the next possible hardware timeout.

This commit adds a comment explaining this.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
This commit is contained in:
Ezequiel Garcia 2015-05-11 14:41:05 -03:00 committed by Wim Van Sebroeck
parent deb8d50eb4
commit c1f263878e
1 changed files with 29 additions and 0 deletions
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29
drivers/watchdog/imgpdc_wdt.c
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@ -9,6 +9,35 @@
*
* Based on drivers/watchdog/sunxi_wdt.c Copyright (c) 2013 Carlo Caione
* 2012 Henrik Nordstrom
*
* Notes
* -----
* The timeout value is rounded to the next power of two clock cycles.
* This is configured using the PDC_WDT_CONFIG register, according to this
* formula:
*
* timeout = 2^(delay + 1) clock cycles
*
* Where 'delay' is the value written in PDC_WDT_CONFIG register.
*
* Therefore, the hardware only allows to program watchdog timeouts, expressed
* as a power of two number of watchdog clock cycles. The current implementation
* guarantees that the actual watchdog timeout will be _at least_ the value
* programmed in the imgpdg_wdt driver.
*
* The following table shows how the user-configured timeout relates
* to the actual hardware timeout (watchdog clock @ 40000 Hz):
*
* input timeout | WD_DELAY | actual timeout
* -----------------------------------
* 10 | 18 | 13 seconds
* 20 | 19 | 26 seconds
* 30 | 20 | 52 seconds
* 60 | 21 | 104 seconds
*
* Albeit coarse, this granularity would suffice most watchdog uses.
* If the platform allows it, the user should be able to change the watchdog
* clock rate and achieve a finer timeout granularity.
*/
#include <linux/clk.h>